73 Chapter 4 Fundamentals of Collision Damage
Copyright Goodheart-Willcox Co., Inc.
Top Impact
The amount of damage caused by a top impact
can vary greatly. The vehicle could roll over. Some
variables in a rollover include the surface that the
vehicle lands on and the degree of rollover. Soft snow
absorbs force, while hard rock deflects the force back
at the vehicle. If the vehicle is airborne and lands on
its roof, it will often suffer enough damage that it will
not be repairable. If the vehicle ends up on its roof
from a side impact, the damage may not be as severe.
Figure 4-32 shows a truck that has rollover damage.
Top impact and roof damage can also be caused by an
object, like a tree, falling on the roof.
Unibody Damage
The terms used to describe unibody damage are
the same as those used to describe full frame damage.
Unibody damage often involves distortion of the
unirails. A shortening in the length of a unirail is called
mash; a change in height is called sag (down) or kickup
(up); movement of one or both unirails to the left or
right is sidesway; and if the rocker panels are not level
with each other, the damage is called twist. Diamond
damage can also occur in a unibody vehicle. In this
type of damage, the center section of the vehicle is not
square, due to an impact on a front or rear corner. A
unibody vehicle with diamond damage is extensively
distorted and is not likely to be repaired.
To help you envision unirail damage, construct a
model as shown in Figure 4-33. Begin by making a fold
across the width of the paper. This fold represents a
convolution or reinforcement in a unirail. Then, fold the
paper lengthwise as shown and tape it to make a box
structure. Push the ends of the box together to simulate
longitudinal force on a unirail. Note that the paper box
distorts in the area of the first fold (convolution or rein-
forcement). The paper box does not mash uniformly.
Instead, it is shorter on one side than the other. The
shortening on one side causes the box to deflect up,
down, or sideways. The box is no longer straight.
This is similar to what happens when longitudinal
force is applied to a unirail. The force of the impact moves
through the unirail until it hits a convolution. The convolu-
tion acts as a pivot point. Buckling takes place at the pivot
point. Any remaining force travels to the next convolution
and buckling again occurs. As force travels through the
unirail, each convolution distorts as it absorbs energy. In
this way, the collision force is kept away from the passen-
gers, if possible. If the force is great enough, distortion
can take place in the passenger compartment.
Goodheart-Willcox Publisher
Figure 4-31. Mash in the arched area caused the
spring to have greater-than-normal arch, causing the
impact side of the vehicle to sit higher off the ground.
Compare the top and bottom of the gap between the
cab and bed.
Goodheart-Willcox Publisher
Figure 4-32. This truck rolled over. Notice the
extensive damage a top impact can do to a cab.
Chief
Figure 4-30. If the impact is not on a cross member,
the cross members may act as pivot points and the
wheelbase may shorten on the impact side.
Impact
force